1. Field of the Invention
The present invention relates to a field winding synchronous generator-motor having an armature winding and a field winding that is mainly mounted on vehicles, and that operates as an electric motor at start-up of an internal combustion engine, as well as operates also as a generator after starting.
2. Description of the Related Art
Recently, for the purposes of an environmental protection or improvement of fuel efficiency, vehicles equipped with an internal combustion engine and the other power source, for example, a generator-motor, being the so-called hybrid vehicles have been developed and practically used. In such vehicles, an internal combustion engine and the other power source are properly used depending on running conditions. For example, to suppress unnecessary fuel consumption at the time of idling, there is the following technique referred to as idle stop. In this idle stop, the internal combustion engines is stopped in the case of being stopped at traffic signals, and then the internal combustion engines is re-started by means of a generator-motor in the case where the intension of starting of a driver such as ON of an accelerator or OFF of a brake pedal is detected.
In a generator-motor mounted onto such vehicles, since it is mounted in the state of giving and receiving torques with respect to an internal combustion engine, the rotational speed thereof is largely varied due to effects of the accelerator operation of a driver or the friction of the internal combustion engine. Therefore, as a type capable of controlling the induced voltage of a generator-motor with no regard to the engine speed of an internal combustion engine, it is not employed the one in which a permanent magnet is contained in a rotor, but employed a field winding type capable of controlling field currents. Furthermore, in respect of efficiency, a three-phase AC synchronous generator-motor is employed.
In this regard, when using a generator-motor as an electric motor, an electric power is supplied to the generator-motor via a power converter functioning to convert DC from a batter mounted on vehicles to AC. However, in the case where an induced voltage that the generator-motor generates is higher than the voltage of a battery, any electric power cannot be supplied to the generator-motor. In general, an induced voltage is proportional to a rotational speed, the number of armature windings and a main magnetic flux. Whereas, since a battery to be mounted on vehicles has drooping characteristics, when current is started to carry to the generator-motor, the inter-terminal voltage of a battery drops substantially in proportion to the amount of current to carry.
Thus, as a method of efficiently utilizing the voltage of a battery having these drooping characteristics, it has been proposed a method of targeting the reduction of switching loss or the improvement in voltage utilization rate by decreasing the number of times of switching of a dc/ac converter, that is, a method of applying rectangular waves having an arbitrary conducting width. Since, however, merely the width of voltage to be applied is controlled, a disadvantage exists in that current cannot be controlled with accuracy. In addition, to control current with accuracy, there is a known technique of making PWM (Pulse Width Modulation) switching of a part of pulse of rectangular waves. However, e.g., a rectifier capacitor is needed in order to make switching, and thus a disadvantage exits in that lower costs and downsizing cannot be achieved.
Also in the past, although there have been proposed many inventions related to a rectangular wave conduction targeting a DC brushless motor, a device for detecting an induced voltage and/or a device for detecting current flowing through an armature is essentially required, thus to be unsuitable for lower costs and downsizing. Moreover, since generally a DC brushless motor is equipped with a permanent magnet at a rotor, an induced voltage to be generated comes to be substantially constant on the conditions of a constant rotational speed. Further, similar techniques are disclosed in the Japanese Patent No. 3183356, the Japanese Patent No. 3333442, the Japanese Patent No. 3574046, and the Japanese Patent Publication (unexamined) No. 153580/1994.
However, in the case of being mounted in the state capable of giving and receiving torques mainly with respect to an internal combustion engine of vehicles as a generator-motor according to the invention, since the rotational speed is largely varied as described above, the induced voltage is largely varied as well, leading to a problem of impaired controllability.